1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus particularly suitable for using as a copying machine, a laser beam printer or the like.
2. Related Background Art
FIG. 7 schematically shows a conventional image forming apparatus. Incidentally, the image forming apparatus shown in FIG. 7 is a laser beam printer having four developing devices which can selectively be switched and an intermediate transfer belt as an intermediate transfer member.
In the image forming apparatus shown in FIG. 7, a surface of a photosensitive drum 1 rotated in a direction shown by the arrow R1 is uniformly charged by a charge roller (charger) 2, and an electrostatic latent image corresponding to an yellow image is formed by illuminating a laser beam from an exposure device 3 having a light source and a mirror. By rotating a rotary 4a of a developing means 4 in a direction shown by the arrow R4, a developing device 4Y corresponding to yellow is brought to a developing position D where the developing device 4Y is opposed to the photosensitive drum 1. In the developing position D, yellow toner is adhered to the electrostatic latent image on the photosensitive drum 1, thereby forming an yellow toner image. The yellow toner image is first-transferred onto an intermediate transfer belt (intermediate transfer member) 5a. The intermediate transfer belt 5a is mounted around rollers 5b, 5c and 5d and is rotatingly driven in a direction shown by the arrow R5, and the yellow toner image on the photosensitive drum 1 first-transferred onto the intermediate transfer belt 5a at first transfer nip T.sub.1 by applying predetermined first transfer bias to a first transfer roller 5e. After the first transferring of the toner image, first transferring residual toner remaining on the photosensitive drum 1 is removed by the cleaner 6, thereby preparing for next magenta image formation.
The series of processes effected regarding the yellow toner (i.e., charging, exposure, developing, first transferring and cleaning) are performed regarding magenta toner, cyan toner and black toner (other than the yellow toner), four color toner images are superimposed on the intermediate transfer belt 5a. The four color toner images are collectively secondary-transferred onto a transfer material P such as a paper sheet at a secondary transfer nip T.sub.2 while the transfer material P supplied from a sheet supply cassette (not shown) by a sheet supply roller (not shown) is being conveyed between a secondary transfer roller 5f (rotated in a direction shown by the arrow Kf) and the intermediate transfer belt 5a, by applying secondary transfer bias to the secondary transfer roller 5f. After the secondary transferring, the transfer material P is sent, by a convey belt (not shown), to a fixing device 7, where the toner images are fixed to the transfer material by heat and pressure. In this way, image formation is finished. On the other hand, after the secondary transferring, secondary transferring residual toner remaining on the intermediate transfer belt 5a is removed by a cleaner 5g, thereby preparing for next first transferring.
Now, the cleaning of the intermediate transfer belt 5a will be explained. The cleaning roller 5g is constituted by a metal core, a base layer coated on the metal core and having a thickness of 2 to 6 mm and made of rubber having resistance of 10.sup.4 to 10.sup.6 .OMEGA..cm, and a surface layer having a thickness of 50 to 300 .mu.m and made of rubber or resin having resistance of 10.sup.6 to 10.sup.12 .OMEGA..cm. The cleaning roller 5g can be shifted (in a direction shown by the arrow Kg) toward and away from the intermediate transfer belt 5a by a drive means (not shown). When the toner images on the photosensitive drum 1 are successively first-transferred onto the intermediate transfer belt 5a, the cleaning roller 5g is disengaged from the intermediate transfer belt 5a, and, after the toner images on the intermediate transfer belt 5a are secondary-transferred onto the transfer material P, the cleaning roller 5g is engaged with the intermediate transfer belt 5a. In this condition, bias is applied from a bias power source 5i to the cleaning roller 5g. Briefly explaining a cleaning method used herein, the secondary transferring residual toner remaining on the intermediate transfer belt 5a is charged with polarity opposite to normal charging polarity of the toner of the developing means 4 by the cleaning roller 5g to which the direct current bias was applied and is transferred from the intermediate transfer belt 5a onto the photosensitive drum 1 at the first transfer nip T.sub.1, thereby cleaning the surface of the intermediate transfer belt 5a. The secondary transferring residual toner transferred to the photosensitive drum is also removed by the cleaner 6 (for the photosensitive drum 1) for removing first transferring residual toner.
However, in the above-mentioned conventional technique, particularly when a four full-color image is formed, since an amount of the secondary transferring residual toner on the intermediate transfer belt 5a is increased, the secondary transferring residual toner cannot be charged adequately with opposite polarity by the cleaning roller 5g, thereby arising a problem that cleaning ability is worsened. Further, when images are continuously formed, there arises a problem that the poor cleaning affects a bad influence upon a next image. On the other hand, if a value of the bias applied to the cleaning roller 5g is increased, the secondary transferring residual toner can be charged adequately. In this case, however, the intermediate transfer belt 5a is charged excessively, with the result that, when the images are continuously formed on a plurality of transfer materials P, the first transferring ability regarding second and other transfer materials P is worsened.
In the above description, while an example that the intermediate transfer belt 5a is used as the intermediate transfer member was explained, if intermediate transfer drum is used, the similar problems will arise.